Well, Tartarus was basically Hell in Greek mythology - it was the lower region of Hades, which was the underworld, which Cerebus guarded, so... yeah, Tartarus is basically pony Hell, that might be able to be reached by the living (though I have a theory that Cerebus guards a kind of portal between Tartarus and the living world).

By that logic, I'm guessing their heaven is Elysium or something (closest thing Greek myths had to Heaven)

Yeah, that's the thing. He had more time to be introduced, and, quite frankly, he was a damned good villain. Although Chrys was more creepy because of her design and talking about having her subjects feed.

Chrysalis has an absolute gorgeous design, but she didn't have much of a history with Equestria like NMM or Discord did. But of all the villains... she's definitely the one that is most intimidating to me.

Tell it like it is, sister. Nightmare Moon was a good concept, but she wasn't the best villain.

To her credit, the show was focusing on introducing everyone for the first episode, so they couldn't really work on the villain that would only appear once. (Twice if you count the episode as two halves.)